1. Field of the Invention
This invention relates to a side member of a motor vehicle, and more particularly to improvements in a side member of a motor vehicle, wherein an impact energy of impact load in the axial direction applied to the side member can be absorbed by an axial compressive deformation.
2. Description of the Prior Art
There are known side members of a motor vehicle, wherein beads arranged in the longitudinal direction are provided, and, when an impact load in the axial direction is applied, an axial compressive deformation is effected to absorb energy of the impact load.
However, generally, these conventional side members have been improper in shape, length of beads or pitch in the axial direction thereof, so that it is difficult to stably effect the axial compressive deformation.
For example, normally a side member forms a closed section of a rectangular form. However, when the beads are formed, crossing the opposing corner portions in the vertical direction on one side surface of the side member, the bending strength of the side member is reduced considerably, such that when an impact load is applied to the side member in the axial direction, the side member is bent before the side member is axially compressively deformed over the scope of the total length thereof. The reason for this disadvantage is that when a bending moment acts on a tetragonal closed-section member, the stress caused by this bending moment is mainly borned by the corner portions of the tetragon. As a consequence, when the beads are formed at the corner portions bearing a large part of the stress, the bending stress must be borne by the other portions, causing the bending strength of the side member to be lowered considerably.
Furthermore, when the pitch of the beads is improper, as shown in FIG. 11 for example, a disadvantage is presented such that an end portion 3 of an expansive deformation portion 2 of the side member 1, which has initiated an axial compressive deformation, irregularly extends in the direction of the deformation, such that a stable axial compressive deformation cannot be obtained.